Tiny lipoprotein sponges: How hen egg yolk high-density lipoprotein microgels hold and release hydrophobic molecules

Protein microgels are gaining increasing attention in various fields, e.g., as carriers for targeted delivery and controlled release of food-grade bioactives. In this research, we investigated the uptake and release behavior of spherical microgels formed from high-density lipoprotein (HDL) extracted from hen egg yolk, using Rhodamine B (Rho B) as a model amphipathic cargo molecule. Microgels were fabricated by structuring HDL within water-in-water emulsion droplets via Au³⁺-mediated crosslinking. Rho B uptake was evaluated as a function of time, initial concentration, pH, and temperature. Uptake increased with longer exposure, higher initial dye concentration, acidic pH, and elevated temperature, with temperature exerting the strongest effect (maximum uptake at 76 °C). Lipid removal significantly reduced Rho B uptake, whereas the neutral hydrophobic dye Nile Red showed no comparable lipid dependence. These findings indicate that uptake is governed not solely by hydrophobic partitioning, but also by electrostatic interactions and temperature-dependent changes in microgel structure and permeability. Release studies conducted in different media revealed that temperature and ionic strength effectively promoted Rho B release, whereas pH had little influence. The microgels exhibited resistance to proteolytic degradation, supporting sustained release behavior. These findings highlight the potential of HDL microgels as carriers for nutraceuticals and other functional ingredients.